The customer reported hearing a strange short duration ghostly whistle sound emanating from their Servis Fridge Freezer evey time they closed the fridge door. ❗ Thorough clearing of the defrost drain tube and the sound was gone. It seems as if the shrinking air when the door was closed was drawing air in through the partially obstructed pipe which acted as a whistle. I never saw this before but I suppose it could happen with any auto defrost model.
tom